Vascular stent with composite structure for magnetic reasonance imaging capabilities

ABSTRACT

A stent is adapted to be implanted in a duct of a human body to maintain an open lumen at the implant site, and to allow viewing body tissue and fluids by magnetic resonance imaging (MRI) energy applied external to the body. The stent constitutes a metal scaffold. An electrical circuit resonant at the resonance frequency of the MRI energy is fabricated integral with the scaffold structure of the stent to promote viewing body properties within the lumen of the stent.

CROSS-REFERENCE TO RELATED APPLICATION

The prestent application is related to U.S. application Ser. No.09/663,896, assigned to the same assignee as the prestent application.

BACKGROUND OF THE INVENTION

Interventional cardiology, interventional angiology and otherinterventional techniques in cardiovascular and other vessels, ducts andchannels of the human body have demonstrated marked success in recentyears. Studies of interventions in the treatment of acute myocardialinfarction (MI), for example, indicate the effectiveness of primaryangioplasty. Implantation of coronary stents has improved the outcome ofsuch interventional treatment. For example, these results are describedin an article in the Journal of American College of Cardiology 2000, 36:1194-1201.

Stents are being implanted in increasing numbers throughout the world totreat heart and cardiovascular disease, and are also coming into greateruse outside strictly the field of cardiology. For example, othervascular interventions utilizing stents which are proving to be of equalimportance to use in cardiology include stenting of the carotid, iliac,renal, and femoral arteries. Moreover, vascular intervention with stentsin cerebral circulation is exhibiting quite promising results,especially in patients suffering acute stroke.

Stents are implanted in vessels, ducts or channels of the human body toact as a scaffolding to maintain the patency of the vessel, duct orchannel lumen. A drawback of stenting is the body's natural defensivereaction to the implant of a foreign object. In many patients, thereaction is characterized by a traumatic proliferation of tissue asintimal hyperplasia at the implant site, and, where the stent isimplanted in a blood vessel such as a coronary artery, formation ofthrombi which become attached to the stent. Each of these adverseeffects contributes to restenosis—a re-narrowing of the vessel lumen—tocompromise the improvements that resulted from the initial re-opening ofthe lumen by implanting the stent. Consequently, a great number of stentimplant patients must undergo another angiogram, on average about sixmonths after the original implant procedure, to determine the status oftissue proliferation and thrombosis in the affected lumen. Ifre-narrowing has occurred, one or more additional procedures arerequired to stem or reverse its advancement.

For virtually all stent implant patients it is desirable to examine andanalyze the patency of the vessel lumen and the extent of tissue growthwithin the lumen of the stent, and to measure blood flow therethrough,from time to time as part of the patient's routine post-procedureexaminations. Current techniques employed to analyze patency of thelumen following a stent implant procedure are more or less invasive.

Among these techniques is vascular puncture, which, despite a relativelylow complication rate, poses inherent risks as well as discomfort of thepatient, such as a need for compression of the puncture site. Use ofiodine containing contrast dye also prestents the possibility ofnegative implication such as renal failure, especially in patients withdiabetes. If contrast dyes are applied to a cerebral perfusion, tissuedamage may cause neurological seizures and temporary cerebraldysfunction. Therefore, it is advantageous to determine the vascularstatus and the functional and morphological capacity of the vascular bedby less or non-invasive methods, including methods not requiringapplication of iodine containing contrast dye.

Fluoroscopic techniques are an unsuitable substitute or alternative forthe invasive methods because the metal stent itself causes blockage ofthe x-rays. Although visualization of the stent is achieved by itsfluoroscopic portrayal as a shadow during the original implantprocedure, the stent's very presence defeats subsequent examination ofthe interior condition of the stent and the vessel lumen at the implantsite by means of fluoroscopy following the implant procedure, withoutthe use of contrast dye applied intravascularly.

Magnetic resonance imaging (MRI) can be used to visualize internalfeatures of the body if there is no magnetic resonance distortion. MRIhas an excellent capability to visualize the vascular bed, withparticularly accurate imaging of the vascular structure being feasiblefollowing the application of gadolinium, a contrast dye which enhancesthe magnetic properties of the blood and which stays within the vascularcirculation. This has special implications for the perfusion in vesselswhich are in a stable and resting state, especially iliac, femoral,carotid, and cerebral perfusion. On occasion of acute cerebrovascularstroke, the diagnosis of a blocked artery can be achieved quickly,within minutes, by means of an MRI technique following the intravenousinjection of 30 milliliters (ml) of gadolinium.

Imaging procedures using MRI without need for contrast dye are emergingin the practice. But a current considerable factor weighing against theuse of magnetic resonance imaging techniques to visualize implantedstents composed of ferromagnetic or electrically conductive materials isthe inhibiting effect of such materials. These materials causesufficient distortion of the magnetic resonance field to precludeimaging the interior of the stent. This effect is attributable to theirFaradaic physical properties in relation to the electromagnetic energyapplied during the MRI process.

It is a primary aim of the prestent invention to provide a stentstructure and method that enables imaging and visualization of the innerlumen of an implanted stent by means of an MRI technique without needfor X-ray or contrast dye application. A related aim is to enableanalysis and evaluation of the degree of tissue proliferation andthrombotic attachment within the stent, and thereby, calculation of theextent of restenosis within the stent, as well as to measure the degreeof blood flow, using only MRI and electromagnetic measurement of bloodflow.

In German application 197 46 735.0, which was filed as internationalpatent application PCT/DE98/03045, published Apr. 22, 1999 as WO99/19738, Melzer et al (Melzer, or the 99/19738 publication) disclose anMRI process for represtenting and determining the position of a stent,in which the stent has at least one passive oscillating circuit with aninductor and a capacitor. According to Melzer, the resonance frequencyof this circuit substantially corresponds to the resonance frequency ofthe injected high-frequency radiation from the magnetic resonancesystem, so that in a locally limited area situated inside or around thestent, a modified signal answer is generated which is represtented withspatial resolution. However, the Melzer solution lacks a suitableintegration of an LC circuit within the stent.

Therefore, it is another significant aim of the prestent invention toprovide a structure which enhances the properties of the stent itself toallow MRI imaging within the interior of the lumen of the implantedstent.

SUMMARY OF THE INVENTION

The prestent invention resides in a stent configuration and method ofuse thereof that allows imaging and visualization of the interior of thelumen of the stent after implantation in a body. Interior structures ofprimary interest and concern consist of body tissue build-up, thrombusformation and the characteristics of blood flow. The imaging is madefeasible by a novel stent configuration which includes a tubularscaffolding structure that provides mechanical support for the vessel,duct or channel wall after the stent is deployed at a target site, andadditional electrical structure which overlies the mechanicallysupportive tubular structure. An electrically inductive-capacitive (LC)circuit which is resonant at the magnetic resonant frequency of the MRIenergy is formed by a predetermined geometric configuration of anelectrically conductive layer overlying the primary mechanicallysupportive layer of the tubular stent structure or scaffolding of lowferromagnetic property. The two layers are separated from one another byan electrically insulative layer. This structure enables imaging andvisualization of the interior of the stent by the non-invasive MRItechnique.

In one of its aspects, then, the invention resides in a stentconstructed and adapted to be implanted in a vessel, duct or channel ofthe human body as a scaffolding to maintain patency of the lumenthereof, wherein the stent comprises a mechanically supportive tubularstructure composed at least primarily of metal having relatively lowferromagnetic property, and at least one electrically conductive layeroverlying at least a portion of the surface of the tubular structure toenhance properties of the stent for MR imaging of the interior of thelumen of the stent when implanted in the body. An electricallyinsulative layer resides between the surface of the tubular structureand the electrically conductive layer. The tubular structure withoverlying electrically conductive layer and electrically insulativelayer sandwiched therebetween are arranged in a composite relationshipto form an LC circuit at the desired frequency of magnetic resonance.The electrically conductive layer has a geometric formation arranged onthe tubular scaffolding of the stent to function as an electricalinductance element and an electrical capacitance element.

In a preferred embodiment of the prestent invention, the tubularscaffolding structure is composed of niobium with a trace amount ofzirconium for added strength. The thickness of this structure ispreferably up to approximately 100 microns (micrometers, or μm). Theelectrically insulative layer is an oxide of the metallic materialcomposing the scaffolding, e.g., a layer of niobium oxide orniobium-zirconium alloy oxide, having a thickness of less than about oneμm, and the electrically conductive layer overlying this insulativelayer is preferably composed of niobium, with a thickness of less thanabout 10 μm. It is important to avoid electro-galvanic potentialsbetween the scaffolding and conductive structures.

The LC circuit integrated within the stent structure according to theprinciples of the prestent invention further reduces the already lowferromagnetic properties of the stent and at the magnetic resonantfrequency, to enhance visualization of body tissue and tissue growthwithin the lumen of the implanted stent during the magnetic resonanceimaging. The LC circuit also enables measurement of the blood flowthrough the lumen of stent implanted in a blood vessel.

The LC circuit is alternatively formed as a bird cage or saddle coilpattern.

BRIEF DESCRIPTION OF THE DRAWING

The above and still further aims, objectives, features, aspects andattendant advantages of the prestent invention will become apparent tothose skilled in the art from the following detailed description of abest mode prestently contemplated of practicing the invention byreference to certain preferred embodiments and methods of manufactureand use thereof, taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a side view of a preferred configuration of the base tubularscaffolding structure of an embodiment of a stent according to theinvention;

FIG. 2 is a highly magnified cross-sectional view through a strut of thestent configuration of FIG. 1;

FIG. 3 is a development side view of a portion of the stent whichillustrates physical formation of the LC circuit on the surface of thestent substrate structure;

FIG. 4 is a schematic illustration of the LC electrical circuit;

FIG. 5 is a diagram illustrating an LC circuit formed using theprinciple of a bird cage; and

FIG. 6 is a diagram illustrating an LC circuit using a saddle coilprinciple.

DETAILED DESCRIPTION OF THE PRESENTLY CONTEMPLATED BEST MODE OFPRACTICING THE INVENTION

FIG. 1 is a side view (not to scale) of a preferred configuration of astent scaffolding structure 10 (albeit this particular configuration isnot esstential to the principles of the invention) which may be employedfor purposes of the invention. The stent has the form of a hollowtubular self-supporting (i.e., mechanically supportive, when implantedand deployed) structure, preferably composed entirely or principally ofniobium. Niobium (Nb) is a lustrous light gray ductile metallic elementthat resembles tantalum chemically and is frequently used in alloys.Like tantalum, niobium is corrosion resistant and non-ferromagnetic, butis formable, weldable and easier to machine. It has mechanicalproperties similar to those of steel, and is highly biocompatible whichmakes it suitable for use in an implant. For enhanced strength andcertain other desirable physical characteristics, a trace amount ofzirconium is added to the niobium prior to forming the material into asolid tubular shape for processing as the scaffold structure of thestent. The percentage by weight of zirconium in the niobiumzirconiumalloy is preferably less than 5%, more preferably less than about 2%,and most preferably less than about 1%, the remainder being niobium. Aniobium alloy stent structure is the subject of the aforementionedrelated co-pending U.S. application Ser. No. 09/663,896. The material ispreferably diamagnetic, but a paramagnetic substrate will also suffice.

This stent composition is non-allergenic, has enhanced radiopacity,offers freedom from distortion on MRI, is flexible with suitableelasticity to be plastically deformable, has good mechanical strength(similar to that of steel) to render the stent scaffold resistant tovessel recoil (as invariably occurs after the stent is deployed at atarget site in the vessel), all of these characteristics or propertiesbeing possessed in a structure sufficiently thin to offer minimalobstruction to flow of blood (or other fluid or material in vessels,ducts, channels or tracts other than the cardiovascular system) by thestent wall. Although a solid tubular structure is preferred (withopenings formed through the sidewall to accommodate expansion of thestent during deployment), other known tubular configurations such aswire mesh and coil configurations may alternatively be used

The tubular scaffold structure of the stent shown in side view in FIG. 1has its far side, as viewed in the Figure, omitted to avoid unnecessaryclutter and confusion in the depiction. The particular configurationillustrated in the Figure is described in greater detail in co-pendingapplication Ser. No. 08/933,627, which is assigned to the same assigneeas this application, but will be described briefly here for the sake ofconvenience to the reader. Scaffolding structure 10 has a multiplicityof through-holes or openings 12 through its wall (sidewall) 15, whichare defined and bounded by a plurality of struts or links 13. Theinterlaced struts and separating through-holes enable expansion of thestent's diameter for deployment at a target site in a vessel of thehuman body during implantation of the stent. Holes 12 may be preciselycut out to form the latticework sidewall 15 using a narrow laser beam ofa conventional laser following a pre-programmed pattern. The materialwhich is removed to form the openings 12 is discarded. In itsconfiguration shown in FIG. 1, the scaffold structure of the stent is ina slightly opened (i.e., the diameter of the structure is expanded orpre-opened) state.

By way of example and not of limitation, the resulting pattern in thelatticework wall 15 constitutes a network of interconnected struts 13 inan optimum orientation predominantly parallel to the longitudinal axis16 of the tube 11, in which none of the struts is oriented substantiallyperpendicular (i.e., transverse) to the stent's longitudinal axis 16. Inthis way, no strut interconnecting any other struts in the latticeworkis oriented to lie completely in a plane transverse to the longitudinalaxis, instead running from one end of the stent to the opposite end.This structure is desirable to provide a very low frictioncharacteristic (or coefficient of friction) of the outer surface 17 ofthe stent, to ease advancement of stent 10 in a vessel, duct, channel ortract to a target site where the stent is to be deployed. The network orlatticework of struts 13 may define a series of longitudinally repeatingcircumferential rows 20 of openings 12, in which each opening has ashape which resembles the outline of a handlebar moustache, or of aDutch winged cap, with each opening bounded by alternating links inwavelets of higher and lower crests in successive rows of eachcircumferential column displaced along the length of the cylindricalelement. If FIG. 1 is viewed upside down, the openings have a shaperesembling the outline of a ram's head with horns projecting at eitherside upwardly from the head and then downwardly, each opening bounded byalternating links in wavelets of shallower and deeper troughs insuccessive rows of each circumferential column displaced along thelength of the cylindrical element.

Each pair of struts such as 21, 22 bounding an opening 12 in any givenrow 25 are in the shape of circumferentially displaced wavelets withadjacent circumferentially aligned higher and lower crests 26, 27,respectively, in which the wavelets intersect one another at one or bothsides of the crests (30, 31). The intersection 30 of struts (orwavelets) at one side of the adjacent circumferentially aligned crests26, 27 of row 25 is tangential to a crest 33 of the immediately adjacentrow 35, and the intersection 31 of struts (or wavelets) at the otherside of those crests is tangential to a crest 37 of the immediatelyadjacent row 38. Interconnecting points such as 40 between the strutsmay be notched to enhance symmetrical radial expansion of the stentduring deployment thereof.

When the stent 10 is crimped onto a small diameter (low profile)delivery balloon (not shown), the adjacent circumferentially alignedcrests of each row move closer together, and these portions will thenfit into each other, as the pattern formed by the latticework of strutsallows substantial nesting together of the crests and bows, whichassures a relatively small circumference of the stent in the crimpedcondition. Such a stent is highly flexible, and is capable of undergoingbending in an inner arc to a small radius corresponding to radii ofparticularly tortuous coronary arteries encountered in some individuals,without permanent plastic deformation.

As the stent 10 is partially opened by inflation of the balloon duringdeployment, the adjacent crests begin to separate and the angle ofdivision between struts begins to open. When the stent is fully expandedto its deployed diameter, the latticework of struts takes on a shape inwhich adjacent crests undergo wide separation, and portions of thestruts take on a transverse, almost fully lateral or perpendicularorientation relative to the longitudinal axis of the stent. Such lateralorientation of a plurality of the struts enables each fully opened cellto contribute to the firm mechanical support of the scaffolding offeredby the stent in its fully deployed condition, to assure a rigidstructure which is highly resistant to recoil of the vessel wallfollowing stent deployment. This particular configuration of the stentstructure, while highly desirable and preferred in the prestentlycontemplated best mode for practicing the invention, is illustrativeonly and not a limitation on or esstential to the principles of theprestent invention.

After or just prior to final processing, the stent is preferablypre-opened after fabrication to relieve stresses. Pre-opening produces astent inner diameter that allows the stent to slide comfortably over theuninflated mounting balloon of the stent delivery system, for ease ofcrimping the stent onto the balloon. Annealing may be performed afterpre-opening by heating the stent structure to an appropriate temperaturefor a predetermined interval of time.

The niobium/zirconium alloy of which the stent is preferably composed isfabricated in any conventional manner, with a percentage by weight ofzirconium amounting from preferably less than about 1%, up to about 5%,and the remainder being niobium. For example, the manufacturing processmay be performed by sintering particles or microspheres of theconstituent metals under heat and pressure. Instead of zirconium as thetrace metal of the primarily niobium alloy, a trace amount (e.g., lessthan one to three percent) of titanium, tantalum or other metal ofsimilar properties, may be alloyed with the niobium for added strengthand other desirable physical characteristics. Other suitable alternativeadditive materials include those described in U.S. Pat. Nos. 5,472,794and 5,679,815, for example. The alloy is then formed into tubing and thethrough holes are provided in its wall by a method such as thepreviously mentioned laser cutting.

According to the invention, two additional layers are adherentlysuper-positioned atop the surface of the scaffolding substrate structure10 to form a composite structure which provides electrical elements orcomponents that gives the final stent its enhanced properties orcharacteristics for enabling magnetic resonance imaging of the interiorof the lumen of the stent when implanted in the body. FIG. 2 is across-sectional view through a strut 13, which is highly magnified forthe sake of clarity of the description. The Figure illustrates theprincipally niobium (pure, or as an alloy with a trace element such aszirconium for strength) substrate 13 of the strut and its overlyinglayers. The latter comprise an electrically non-conductive, orinsulative, layer 51, and an electrically conductive layer 50 formedatop and adherent to the insulative layer. The insulative layer 51 isadherent to the underlying surface 52 of the strut which, itself, is cutor otherwise formed from the wall or sidewall 15 of the original tubefrom which the stent is fashioned.

The two layers 50 and 51 are confined to preselected portions of thestent substrate surface. Preferably, these layers are applied to orformed upon the outer surface of the stent, rather than to or upon theinner surface along the lumen of the stent. The more practical reasonfor this is that the manufacturing process is more easily performedusing the outer surface location. More importantly, placement at theinner surface of the lumen of the stent would adversely affect thecharacteristics of blood flow (or flow of other fluid in other ducts)through that lumen when the stent is implanted in a body vessel. The twolayers—electrically conductive layer 50 and insulative layer51—contribute little or nothing to the mechanical properties of thestent, but provide important features in magnetic resonance imaging ofthe implant region.

The physics and basics of magnetic resonance imaging (MRI) are wellknown, so only a summary will suffice here. An external magnetic fieldinduces a spin in the atomic nuclei, which is a function of thedirection, strength and change in the externally applied magnetic field.The spin of the atomic nuclei consists of several signals, which can beseparated and described by different relaxation times t1 and t2. Certainmathematical methods are used to recover or receive a signal outside thebody which is proportional to the structure of the material, especiallyof the tissue, in the human body subjected to the MRI procedure.

These mathematical methods consider the total magnetic resonancefrequency, the total magnetic energy, and the gradient between thedifferent relaxation times, and, together with the use of contrast dyeswhich change the paramagnetic properties of the tissue, enable an imageto be created according to the external application of the magneticresonance energy. The imaging creates three-dimensional pictures of notonly external structures of the body, but of virtually any region withinthe body subjected to the MRI energy. An obstacle for creation of acomplete and accurate 3-D picture is any metallic implant in thepatient's body, since this operates to produce a distortion of the MRIimaging, depending in part on the implant's ferromagnetic properties.

The externally applied magnetic resonance imaging energy may beamplified, and a spatial resolution achieved, by use of aninductive-capacitive circuit—an LC circuit—at the magnetic resonancefrequency, as pointed out in the aforementioned 99/19738 publication.

In the device and method of the invention, a structure which forms thesimple electrical circuitry of a spool to achieve amplification of theexternally applied magnetic energy of the MRI apparatus is achieved bythe conductive and insulative layers 50 and 51, respectively, providedon the scaffolding structure of stent 10, so that the LC circuit isintegrated into the stent itself. The sidewall 15 of stent 10, andhence, the strut wall 13 itself, has a thickness in a range of 100microns or less in the case of a coronary stent, for example. Thetypical coronary artery in which a stent is implanted has a diameter ina range of from 2 to 5 millimeters (mm). A stent which is to beimplanted in vessels of larger diameter may and typically would have athicker wall.

The electrical insulation layer 51 is preferably an oxide of the metalthat forms the stent. In the preferred embodiment, the stent scaffoldingstructure or substrate is composed of pure niobium or an alloy ofniobium with a trace of a strengthening element such as zirconium;hence, the layer 51 is preferably niobium oxide or niobium-zirconiumoxide.

Electrically conductive layer 50 overlying the insulative layer ispreferably composed of niobium, has a thickness considerably less thanthe thickness of the stent wall, and a width which preferably is lessthan the width of the underlying strut 13. In a preferred exemplaryembodiment, the electrically conductive layer 50 has a thickness lessthan 10 microns, and a width from about 80 to about 100 microns, whichin any event is not greater than the width of the strut.

FIG. 3 is a development side view of a portion of the stent whichillustrates physical formation of the LC circuit on the surface of thestent substrate, and FIG. 4 is a schematic illustration of the LCelectrical circuit 68. Unlike the prior art, the LC circuit isintegrated within the stent itself. The physical structure of the LCcircuit is determined or calculated to be resonant at the magneticresonance frequency of the MRI energy. This will allow the MRI image todepict the region within the lumen of the stent, as well as the regionexternal to the stent which would ordinarily be viewable by MRI, withoutsignificant distortion.

The black bars 60 (transverse to the longitudinal axis of the stent) inFIG. 3 represtent temporary mask locations over the outer surface 17 ofthe stent (as opposed to the inner surface which constitutes the surfaceof the lumen of the stent). As shown in this Figure, the coil portions63 are formed in a predetermined pattern on outer surface 17. This isachieved by first depositing (or otherwise creating, such as by heatingthe stent in an atmosphere of oxygen to form an oxide of the underlyingmetal, e.g., niobium, in other than surface masked regions) the thinelectrically insulative layer 51 (such as niobium oxide) on the externalsurface 17 of adjacent struts 13 in a circumferential row 25, in acontinuous lineal pattern. Longitudinal extensions 65 of mask bars 60indicate struts 66 which are to be left free of an insulative layer 51and where, instead, that layer is to extend lineally by jumping tostruts in the next adjacent circumferential row 35 above (or below,depending on the vantage point) the relevant mask extension 65. Thecorresponding ends 67 of longitudinally adjacent bars 60 are displacedor offset to terminate below (or above, depending on vantage point) thehigher crest 26 of wavelets in the adjacent rows to leave a one-crestgap 70 between confronting ends of circumferentially adjacent bars 60.This leaves a longitudinally stepped gap at which the jump in linealapplication of insulative layer 51 is to be made. In effect, the maskcreates a map for application of this strip. The previously describedelectrically conductive layer 50 is then formed by application (e.g., bydeposition) directly atop and adherent to insulative layer 51.

With two circumferentially aligned mask bars 60 in each row of struts,this process results in two sub-coils which are then connected atadjacent ends to form a single continuous coil 72 (FIG. 4). The opposingends of this overall coil 72 are effectively coupled together through acapacitance element 73 which is created by the close separation betweenthe two sub-coils in application pattern of the conductive layer 50 onthe stent. As a result, the LC circuit 68 is formed as an integral partof stent 10. The geometry of the LC circuit including the length of thecoil and the capacitance produced by the spacing between the adjacentsub-coils is predetermined to achieve the desired magnetic resonancefrequency.

As an alternative to this technique or method of forming LC circuit 68,the principle of a bird cage may be used, as illustrated in FIG. 5. Thebird cage 75 has longitudinal elements 77, which are formed by applyingstrips of a conductive layer 50 overlying strips of an insulative layer51 atop the outer surface 17 of a series of longitudinally aligned andinterconnected struts from end to end of the stent, as described above,except for a break or interruption 78 at a central point of eachlongitudinal element. At each end 79, 80 of the stent the correspondingends of these longitudinal elements 77 are connected together by arespective transverse circumferential connecting strip 82,83 of theconductive layer overlying a similarly situated insulative layer atopthe outer surface 17 of the stent. The result is an integral LC circuitas in FIG. 4, having a magnetic resonance frequency determined accordingits geometry.

Another alternative form that provides LC circuit 68 is shown as aso-called saddle coil 85 in FIG. 6. In this example, four conductivelongitudinal elements 87 are created on four series of longitudinallyaligned and interconnected struts from end to end of the stent, asdescribed above but without central interruption. Two sets of two eachof the elements 87 residing at 120 degree separation are connectedtogether at one end of the stent by partial circumferential endconductive elements 88, 89, respectively. Each of these two sets isseparated at opposite sides circumferentially of the stent by 60 degreegaps 91, 92. At the other end of the stent, two of the adjacentconductive elements 87 residing at 60 degree separation (e.g., separatedby gap 91) in one pair of opposite ones of the two sets are connectedtogether by partial circumferential end conductive element 94. Thespacing between the longitudinal elements of the two sets creates aneffective capacitance 95 between elements 87 of the other pair ofopposite ones of the two sets.

Fabricating the electrically conductive and insulative layers atop thescaffolding or substrate mechanical structure may be performed asdescribed above. To avoid galvanic potentials, it is preferable that themechanical structure and the electrically conductive structure shouldconsist of materials of similar electro-galvanic potential, and, in theextreme, composed of materials from the same metallic group. A mask(e.g., a traditional mask including photo-resist or otherwise) may beapplied to the substrate structure, the insulative (e.g., oxide) layerand the overlying conductive layer may be formed by sputtering or vapordeposition or other known techniques for applying a metal or othermaterial to a preexisting structure under vacuum and electrical highenergy fields. Alternatively, the entire outer surface of the stentscaffolding structure may be covered by layers of insulation (oxide) andconductive material, after which selected portions may be removed, as byknown laser removal techniques.

The geometric structures are created and defined by the use of anappropriate mask. The resonant frequency of the inductive-capacitivecircuit structure may be adjusted as desired according to the geometricconfiguration of the outer conductive layer atop the insulative layer.

Although a best mode of practicing the invention has been disclosed byreference to certain preferred embodiments and methods, it will beapparent to those skilled in the art from a consideration of theforegoing description, that variations and modifications may be madewithout departing from the spirit and scope of the invention.Accordingly, it is intended that the invention shall be limited only bythe appended claims and the rules and principles of applicable law.

What is claimed is:
 1. A stent adapted to be implanted in a duct of ahuman body to maintain an open lumen at the implant site, and to allowviewing body properties outside and within the implanted stent bymagnetic resonance imaging (MRI) energy applied external to the body,said stent comprising a metal scaffold, and an electrical circuitresonant at the resonance frequency of said MRI energy integral withsaid scaffold.
 2. A stent adapted to be implanted in a duct of a humanbody to maintain an open lumen at the implant site, said stentcomprising a tubular scaffold of low ferromagnetic metal, and aninductance-cpacitance (LC) circuit integral with said scaffold, said LCcircuit being geometrically structured in combination with said scaffoldto be resonant at the resonance frequency of magnetic resonance imaging(MRI) energy to be applied to said body to enable MRI viewing of bodytissue and fluid within the lumen of the stent when implanted andsubjected to said MRI energy.
 3. The stent of claim 2, wherein saidmetal is at least primarily niobium.
 4. The stent of claim 3, whereinsaid LC circuit is composed of successive layers of an oxide of saidmetal overlying said scaffold and the same composition as said metaloverlying said oxide layer.
 5. A stent constructed and adapted to beimplanted in a vessel, duct or channel of the human body to maintainpatency of the lumen thereof and to allow substantially distortion-freemagnetic resonance imaging (MRI) of the interior of the stent whenimplanted and subjected to MRI energy, said stent comprising a tubularscaffolding structure composed at least primarily of metal havingsufficiently low ferromagnetic property to avoid material distortion ofthe magnetic resonance field, at least one electrically conductive layeroverlying at least a portion of the surface of said scaffoldingstructure to enhance properties of the stent for magnetic resonanceimagine (MRI) of the interior of the stent when implanted, and anelectrically insulative layer residing between said at least a portionof the surface of said scaffolding structure and said at least oneelectrically conductive layer, wherein said electrically insulativelayer is formed from an oxide of the metal of said scaffoldingstructure.
 6. The stent of claim 5, wherein said tubular scaffoldingstructure is composed at least primarily of niobium and saidelectrically insulative layer is composed of niobium oxide.
 7. The stentof claim 5, wherein said tubular scaffolding structure is composed atleast primarily of niobium zirconium alloy and said electricallyinsulative layer is composed of niobium zirconium oxide.
 8. The stent ofclaim 5, wherein said scaffolding structure and said electricallyconductive and insulative layers are geometrically arranged in acomposite relationship to form an inductance capacitance (LC) circuithaving a resonant frequency to enable visualization of body tissue andtissue growth within the lumen of the implanted stent by MRI.
 9. Thestent of claim 8, wherein the resonant frequency of said LC circuit ismade by said geometrical arrangement to be equivalent to the resonancefrequency of the MRI energy to which the stent is to be subjected in thebody.
 10. The stent of claim 8, wherein said LC circuit is formed toenable measurement of blood flow through the lumen of said stent whenimplanted in a blood vessel of the body.
 11. The stent of claim 8,wherein said LC circuit is formed as a bird cage element.
 12. The stentof claim 8, wherein said LC circuit is formed as a saddle coil element.13. A stent constructed and adapted to be implanted in a vessel, duct orchannel of the human body to maintain patency of the lumen thereof andto allow substantially distortion-free magnetic resonance imaging (MRI)of the interior of the stent when implanted and subjected to MRI energy,said stent comprising a tubular scaffolding structure composed at leastprimarily of metal having sufficiently low ferromagnetic property toavoid material distortion of the magnetic resonance field, at least oneelectrically conductive layer overlying at least a portion of thesurface of said scaffolding structure, and an electrically insulativelayer residing between said at least a portion of the surface of saidscaffolding structure and said at least one electrically conductivelayer, said scaffolding structure and said conductive and insulativelayers being geometrically arranged in a composite relationship to forman inductance capacitance (LC) circuit integrated within the stent forresonance at the magnetic resonance frequency of the MRI energy so as toenable the MRI image to depict the region within and external to thestent without material distortion.
 14. The stent of claim 13, whereinsaid at least one electrically conductive layer has a geometricformation arranged to function as an electrical inductance element. 15.The stent of claim 13, wherein said at least one electrically conductivelayer has a geometric formation arranged to function as an electricalcapacitance element.
 16. The stent of claim 13, wherein said at leastone electrically conductive layer is structured to function electricallyas both an inductance element and a capacitance element.
 17. The stentof claim 13, wherein said at least one electrically conductive layer hasa thickness of less than about 10 μm.
 18. The stent of claim 13, whereinsaid tubular scaffolding structure has a thickness considerably greaterthan the thickness of said at least one electrically conductive layer.19. The stent of claim 13, wherein said at least one electricallyconductive layer has a thickness of less than about 10 μm, and saidelectrically insulative layer has a thickness of less than about 1 μm.20. The stent of claim 3, wherein said at least one electricallyconductive layer is spaced apart from the surface of said tubularstructure by the thickness of said electrically insulative layer.